setting static max = dynamic min = dynamic max did not fix it

a toolstack restart on all hosts in the cluster seems to have resolved the issue, I have not had a chance to dig in to find a root cause yet

This looks like it was a problem with the VDI's on a network shared storage.
Unpausing the VM's would "start" them but they never actually boot - no console would appear.

In the end, migrating all other VM off the host and then rebooting the host cleared whatever was causing these VMs to be stuck and we were able to delete them

We're currently trialing a new storage provider, so this is definitely something we'll be looking into more with their support.